The present invention relates to decentered optical systems and, more particularly, to a compact decentered optical system in which at least three reflecting surfaces having an image-forming power required for image formation are decentered.
Hitherto, an optical system in which a rotationally asymmetric curved surface having no plane of symmetry, e.g. a free-form surface, is three-dimensionally decentered has been proposed by the present applicant in Japanese Patent Application Unexamined Publication (KOKAI) No. 10-186237.
In the conventional three-dimensionally decentered optical system as disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 10-186237, when a reflecting surface is formed by using a back coating mirror, the center points in the effective areas of the entrance, reflecting and exit surfaces of a prism or some points thereof are in one plane. Therefore, it is impossible to freely lay out large reflecting and transmitting surfaces owing to the interference between the effective apertures of the surfaces.